Dead brittle stars on a still-damaged sea floor about 10 miles north of the BP oil rig accident seen from submarine Alvin on Dec. 1, 2010. “We consistently saw dead fauna (animals) at all these sites,” said Marine biologist Samantha Joye of the University of Georgia. “It’s likely there’s a fairly large area impacted,” she said.

Samantha Joye/University of Georgia/AP/File

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A year after the worst off-shore oil spill in US history began, scientists continue to sample the sea floor and comb beaches and marshes in an attempt to track the fate of oil that spewed for nearly three months from the Deepwater Horizon blow-out.

But the emphasis has long since shifted from debates over a simple "Where's Waldo" kind of accounting, to measuring how the oil degrades with time in various environments, and the effects the remnant chemicals can have on the habitats they encounter.

The effort should give restoration teams a better sense of where they need to focus their work. It should also provide benchmarks to more effectively gauge the resilience of land and ocean organisms after their assault by oil and the dispersants used to break up slicks.

The BP spill "gave nature a stress test," says marine scientist Christopher Reddy of the blow-out's environmental aftermath. In trying to assess how nature is responding, it's vital to know how the chemical agents responsible for the stress change over time, he says.

Accounting for the oil

Shortly after BP capped the blow-out on July 14, and oil stopped flowing, federal officials released a checkbook-like accounting of the oil's fate. But it proved controversial.

The uncertainties in some of the categories were large. The report had not undergone a rigorous peer review. Indeed, the numbers were never meant to represent a rigorous, definitive accounting, according to a white paper prepared by the staff of the National Commission on the BP Deepwater Horizon Spill and Offshore Drilling.

Instead, the results intended to give the National Incident Command an general idea of how to allocate clean-up resources.

Chemical dispersants, evaporation and natural degradation had taken their own toll on nearly half of the oil released, according to the document, but scientists pointed out that while such processes can reduce the volume of hydrocarbons in the ocean, it also can leave what's left in a form that takes far longer to break down.

Finally, 26 percent of the oil released fell into the balance sheet's "residual" category – oil washed ashore or trapped in sand or sea-floor sediments, for instance.

In November, the technical group released a 217-page document – a final estimate of the situation as of July 14 – that spelled out in detail the approaches it used to reach its estimates.

A full, detailed partitioning of the oil's fate may never appear, researchers say. Currents and bacteria have long since dissipated once-coherent plumes of oil or gases such as methane.

Effects of the oil trapped in marshes, mud, and sand

It's the balance-sheet's "other" category -- oil in marshes, beaches, and sea-floor sediment -- that many scientists are tracking and analyzing on a regular basis.

They want to assess the oil's current effects on organisms, and furthermore determine what hazards remain for plants and animals – whether the oil continues to degrade or remains as a lump of tar for decades.

Oil and its residues contain several hundred chemical compounds in varying concentrations and with different environmental effects, says Dr. Reddy, who works at the Woods Hole Oceanographic Institution in Woods Hole, Mass. Knowing those mixtures can help determine whether removing any remaining reside could be more damaging to plants, fish, animals in the area than the blow-out leftovers themselves.

One location that has proven tough to assess and treat is the surf zone, where mats of oil can gather in natural trenches between sand bars. It's a tough place to try to hold a boat stable and gather detailed measurements, let alone clean up, according to report issued on February by a scientific advisory panel.

For his part, Reddy and a colleague have been combing beaches in Florida, Alabama, and Mississippi for tar balls, which they bring back to the lab for analysis.

"I know what the oil looked like when it came out of the pipe," he says. "Now I want to know what it looks like a year later."